Recent developments in the field of printing plate precursors concern the use of imageable layer compositions that can be imaged by means of lasers or laser diodes. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the imageable layer compositions are sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using imageable layer compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the imageable layer are hardened and non-exposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the non-exposed regions become an image.
Independently of the type of imageable element, lithography has generally been carried out using a metal substrate (or “support”) such as a substrate comprising aluminum or an aluminum alloy of various metallic compositions. The surface of the metal sheet is generally roughened by surface graining in order to ensure good adhesion to a layer, usually an imageable layer, that is disposed thereon and to improve water retention in non-imaged regions during printing. Such aluminum-supported imageable elements are sometimes known in the art as precursors to planographic printing plates or lithographic printing plates.
To prepare aluminum-containing substrates for lithographic elements, a continuous web of raw aluminum can be treated, for example, using the sequence of steps that includes some type of graining, anodization using a suitable acid to provide an anodic oxide coating, and a post-treatment section that applies a hydrophilic coating before the web is rewound or passed on to coating stations for application of imageable layer formulations.
In the anodization section, the aluminum web is treated to form an aluminum oxide layer on its surface so it will exhibit a high degree of mechanical abrasion resistance necessary during the printing process. This oxide layer is already hydrophilic to some degree, which is significant for having a high affinity for water and for repelling printing ink. However, the oxide layer is so reactive that is can interact with components of the imageable layer in the imageable element. The oxide layer may partially or completely cover the aluminum substrate surface.
In the post-treatment section, the oxide layer is covered with a hydrophilic protective layer (also known in the art as a “seal”, “sublayer”, or “interlayer”) to increase its hydrophilicity before one or more imageable layer formulations are applied. A suitable interlayer may also ensure that during development, the soluble regions of the imageable layer are easily removed from the substrate, leaving no residue and providing clean hydrophilic backgrounds. The hydrophilic interlayer can also protect the aluminum oxide layer against corrosion during development with highly alkaline developers and from dye penetration from the imageable layer.
A variety of substances have been described for this purpose including a silicate, dextrin, calcium zirconium fluoride, hexafluorosilicic acid, and polymers having functional groups such as carboxy, sulfonic acid, phosphonic acid, mercapto, hydroxyl, or amine groups. Specific hydrophilic protective layers are prepared from formulations including poly(vinyl phosphonic acid) (PVPA), vinyl phosphonic acid/acrylic acid (VPA/AA) copolymers, and poly(acrylic acid) (PAA).
Alternatively, the grained and anodized aluminum can also be treated with a phosphate solution that may further contain an inorganic fluoride (PF).
Various aqueous alkaline compositions (developers) are known for processing imaged negative-working and positive-working elements to provide lithographic printing plates. For example, high pH developers containing 5-30% alkali and 0.1-10% of an ethylene oxide/propylene oxide block copolymer are described in U.S. Pat. No. 4,945,030 (Turner et al.), and other developers containing a thickener such as glycerin and a SiO2 to M2O weight ratio of at least 0.3 are described in U.S. Pat. No. 5,851,735 (Miller et al.).
High pH developers may also include various surfactants, anti-foaming agents, suspension agents including alkyleneoxide compounds and sugars as described for example in U.S. Pat. Nos. 5,670,294 (Piro) and 7,147,995 (Takamiya).
During use, the volume and activity of developers are diminished. The processing elements carry out measurable volumes of the developer from the developing apparatus, and the activity of the developer diminishes as it acts on the imaged elements.
Replenishers are often used to replace lost developer volume but these solutions generally have about the same composition and activity as the developers. They usually do not “replace” or restore developer activity. Regenerators are usually used for that purpose. They are designed with a specific combination of chemicals to replace both volume and activity in the “seasoned” developer.